Q &A Sessions

Question 1

What is the importance of a sub-maximal effect of [Ca2+]i in cardiac myocytes, how does this differ to skeletal muscle?

Answer 1

1. The strength of contraction in cardiac myocytes is proportional to the amount of calcium inside the cell during depolarisation.

This isn’t how the force of skeletal muscle is regulated, in this case the strength of contraction is regulated by recruiting more or less muscle fibres to contract, the more fibres then the stronger the contraction.

Question 2

Recall the equation for blood velocity

Answer 2

blood velocity= blow flow/total cross sectional area

Question 3

What is systemic blood pressure?

Answer 3

This is really the arterial pressure from the left side of the heart. According to Darcy’s law we would subtract the venous pressure from it but since that is only a few mmHg it is usually neglected in this regard.

Question 4

What is pulse pressure?

Answer 4

is the difference between systole and diastole

Question 5

What is mean blood pressure?

Answer 5

related to the systemic blood pressure and is roughly calculated as… Mean BP = diastolic pressure + 1/3 (sytolic - diastolic)

The mean BP gives us some idea of organ perfusion and we are looking for over 70mmHg

Question 6

Which type of pressure gives some idea of organ perfusion?

Answer 6

mean blood pressure

Question 7

What is pulmonary pressure?

Answer 7

driven by the right side of the heart. The pressures here are much lower

Question 8

Why is pulmonary pressure so low?

Answer 8

the top of the lungs are only a few centimetres above the heart so a high pressure isn’t needed to reach them, which saves a lot of energy. Also there are so many capillaries in the lungs for gas exchange that high pressures would lead to too much filtration of fluid from the capillaries leading to pulmonary oedema which interferes with gas exchange.

Question 9

What are typical pressures for blood pressure?

Answer 9

15-30 mmHg systolic and around 4-12 mmHg diastolic.

Question 10

What is central venous pressure?

Answer 10

the pressure returning to the right side of the heart. This is really a measurement of cardiac filling and is important in Starlings law.

Question 11

What happens if the veins constrict too much?

Answer 11

If the veins constrict a bit and send more blood back to the heart we get an increase in central venous pressure
2. then an increase in pre-load

3.then increased cardiac output.

Question 12

What do low CVP and high CVP indicate?

Answer 12

Low CVP may indicate hypovolaemia (or low blood volume)

2.elevated CVP indicates right ventricular failure or volume overload.
The normal CVP range is around 2 – 8 mmHg.

Question 13

What is the result of mitral valve regurgitation?

Answer 13

1. during ventricular systole some of the blood is actually pushed back up into the atrium and not out through the aortic valve.
2. The left ventricle is going to hypertrophy in order to try to increase cardiac output, the heart is having to work increasingly hard just to maintain enough blood going into the aorta.
3. As the heart struggles to work harder its oxygen demand increases.

Question 14

What leads to thicker heart wall in chronic high blood pressure?

Answer 14

1. the heart is being stretched all the time which increases afterload.
2. In order to increase CO and at the same time reduce wall stress, the wall of the heart increases in thickness.
3. The thicker heart wall requires even more oxygen and so the heart has to work harder to pump more blood around itself, but of course this increases the oxygen demand

Question 15

Why during dynamic exercise there is lower sympathetic tone?

Answer 15

In dynamic exercise there has to be a lower sympathetic tone otherwise the increased cardiac output would send blood pressure really high.

Question 16

Why does CVP increase when we are lying down?

Answer 16

It’s because less blood pools in the lower legs, so it more easily returns to the heart. CVP is an approximation of the pressure at the right atrium.

Question 17

Why does aortic pressure drop when aortic valve closes?

Answer 17

As the valve closes there is a notch in the pressure profile of the aorta. This is called the ‘dicrotic notch’ and represents the closing of the aortic valve.

As the valve closes blood flows back towards the heart slightly for a fraction of a second before the walls of the aorta recoil and maintain the aortic pressure

Question 18

How does pressure and radius affect afterload?

Answer 18

Increased blood pressure will put more stress on the walls of the heart and this will oppose contraction

a greater radius means that more of the stress in the heart wall is directed along the wall rather than having a component to the centre of the heart so a greater radius also increases afterload.

Question 19

How does cross sectional area affect tension?

Answer 19

a larger cross sectional area (or width) of the muscle wall distributes the tension over a larger area and therefore means lower stress and less afterload.

S=Pr/2w

Question 20

Why are there spontaneous action potentials in the SA node?

Answer 20

1. it contains voltage gates sodium channels which open at normal resting potentials such as -50mV.
2. these unusual sodium channels make the resting potential unstable and cause another action potential to happen without any other input so we have a pacemaker effect

Question 21

Does β1 stimulation increase both systole and diastole?

Answer 21

you get a faster and harder systole, but the relaxation is also faster
2.so you maintain the diastolic time as more or less the same.

Question 22

What does a low and high compliance mean?

Answer 22

a lower pulse pressure and reducing compliance means a higher pulse pressure.

Question 23

What controls solute diffusion rates?

Answer 23

Blood flow, fall in concentration, Recruitment of capillaries( the more capillaries you have the closer it is so A and X is increased)

Question 24

What is a proxy for stroke volume?

Answer 24

Central venous pressure used as a proxy for stroke volume